A critical role for gastric xanthine oxidoreductase in the formation of S-nitrosothiols and blood pressure responses to nitrite in rats

Nitrite and nitrate bioactivation to nitric oxide (NO) in the enterosalivary cycle of nitrate offers an enormous therapeutic potential for cardiovascular and metabolic diseases and involves xanthine oxidoreductase (XOR), an enzyme widely expressed in the gastrointestinal system. XOR has nitrite reductase activity, reducing nitrite to NO, especially under low pH conditions. Here, we hypothesized that XOR activity critically interacts with acidic gastric pH to cause the blood pressure responses to oral nitrite. To test this hypothesis, rats pretreated with the XOR inhibitor allopurinol (100 mg/kg, by gavage, or vehicle) were treated with N⍵-nitro-l-arginine methyl ester; 60 mg/kg, i.v.) to induce acute hypertension, and received sodium nitrite (1, 5, and 15 mg/kg) in the stomach or in the duodenum. Mean arterial blood pressure (MAP) was monitored invasively. Ozone-based reductive chemiluminescence assays were performed to evaluate plasma nitrite, nitrate, total nitrosylated species and S-nitrosothiols (RSNO) concentrations. Gastric pH was assessed and XOR activity in the stomach and duodenum was assessed by fluorimetry. We found more profound MAP responses to nitrite administered in the duodenum than in the stomach. Importantly, XOR inhibition completely blunted nitrite-induced RSNO formation and hypotensive responses when nitrite is administered in the stomach, whereas XOR inhibition did not affect the responses when nitrite was administered in the duodenum, even though XOR activity is much higher in the duodenum than in the stomach. These results suggest a critical interaction between XOR activity and gastric acidity in mediating RSNO formation and the cardiovascular effects of oral nitrite. These results may help to design better oral nitrite therapeutic formulations.